Many prior art speed control circuits for D.C. motors using SCR's are unsuitable for applications where the motor is used to power a tool for quality cutting of material. This is because, at certain speed settings, the circuit goes into a pulsing mode wherein torque is applied at spaced time intervals and results in a vibratory hammering action of the cutter which produces poor quality cuts and spoilage of material. In hand-held tools, this vibration makes it difficult to hold the tools in a firm position which merely compounds the difficulties.
An analysis of this problem has shown that best results are obtainable by using full-wave circuits and making sure that an alternate SCR is triggered during every half cycle at the same phase angle with respect to the A.C. line voltage or does not trigger at all. Further, it is essential that the SCR's having triggered during any given half cycle, should not trigger again until the proper phase angle is reached during the next half cycle.
Insofar as I am aware, no prior art circuit provides means for functionally satisfying the above-defined criterion.